1. Field of the Invention
The present invention relates to an apparatus and method for obtaining symmetrical junctions between a read sensor and hard bias layers and, more particularly, to an ion beam sputtering system which makes hard bias layers on each side of a read sensor substantially the same size even though they are located near an outside periphery of a wafer substrate during fabrication.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm above the rotating disk and an actuator arm. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals. The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
First and second hard bias layers and first and second lead layers are connected to first and second side surfaces of the sensor, which connection is known in the art as a contiguous junction. This junction is described in commonly assigned U.S. Pat. No. 5,018,037 which is incorporated by reference herein. The first and second lead layers are for the purpose of conducting the aforementioned sense current through the sensor parallel to the ABS and parallel to the major thin film surfaces of the layers of the sensor. The first and second hard bias layers longitudinally stabilize the magnetic moment of the free layer of the sensor in a single domain state. This is important for proper operation of the sensor. It is also important that the first and second hard bias layers be of the same size which is referred to in the art as being symmetrical. If the asymmetry between the first and second hard bias layers is significant, such as one of the hard bias layers being 40% thinner than the other hard bias layer, there may be an insufficient magnetic field between the first and second hard bias layers to longitudinally stabilize the free layer. This problem is prevalent in present methods of forming the junctions between sensors and hard bias layers.
Rows and columns of magnetic head assemblies, wherein each magnetic head assembly includes a read head and a write head, are fabricated on a wafer substrate in a sputtering chamber. Each magnetic head assembly is located on a respective slider wherein each slider is a portion of the wafer substrate. After completing the magnetic head assemblies the wafer substrate is diced into rows of sliders and each row of sliders is then lapped to form the aforementioned ABS. Each row is then diced into individual sliders wherein each slider has a magnetic head assembly with sensitive elements of the assembly exposed at the ABS.
The read sensors of the read head and the hard bias and lead layers are fabricated on the wafer substrate within the aforementioned sputtering chamber. The rows and columns of sliders, where the sensor and hard bias and lead layers are to be constructed, are typically located within a square or rectangle on the wafer substrate. The wafer substrate itself is typically circular. The preferred sputtering system employs a target of the material to be sputtered and an ion beam gun which directs an ion beam onto the target. This then causes atoms to be sputtered from the target and deposited on the wafer substrate. Before sputtering the atoms of the desired material, however, the sensor material layers including the pinned, spacer and free layers are deposited over the entire wafer substrate. A bilayer photoresist mask is then formed at each magnetic head assembly site for defining the side surfaces of each sensor. While the substrate is rotated ion milling is employed for removing all of the sensor material layers except sensor material layer portions which are masked by the photoresist masks wherein each mask may be a bilayer mask or a single mask. The sensor material layer portion below each bilayer photoresist constitutes a read sensor with first and second side surfaces which may be sloping. The next step is to connect the hard bias and lead layers to the first and second side surfaces of each sensor.
With the photoresist masks still in place a target of hard bias material is then bombarded with an ion beam from the ion beam gun causing hard bias atoms to be deposited on the wafer except where the photoresist masks are located. A seed layer is typically formed before the hard bias material is formed. During this process the wafer substrate is rotated. Unfortunately, in prior art sputtering schemes, the photoresist masks near the outer periphery of the wafer substrate cause a shadowing of the deposition at the outer extremities of the photoresist masks. Even though the wafer substrate is rotated during deposition, the hard bias layers are deposited to a desired thickness at the side surfaces of the sensor in a central location of the wafer while the hard bias layers at outer side surfaces of the sensors at outer extremities of the wafer are significantly thinner. This then causes the aforementioned improper biasing of the free layer which can render the read head inoperable for its intended purposes. This problem is not as acute for photoresist masks inwardly from the outer photoresist masks and is practically nonexistent for photoresist masks at a center location of the wafer substrate.
In the prior art, the target, which may be a hard bias or lead sputtering material layer, has a sputtering surface where atoms of the material are to be sputtered and the wafer substrate has a forming surface with a site where the sputtered layer is to be formed by the sputtering. The target has a sputtering center which is located at a center of the atoms to be sputtered from the target and the site has a periphery with a forming center which is located at the center of the periphery. In the prior art the sputtering center is opposite the forming center so that a center of a beam of atoms from the sputtering center hit the forming center of the site. This relationship is modified by the present invention.